Painted articles have varying degrees of surface roughness or irregularity which may be unacceptable to users of the articles. Automotive vehicles and appliances, for example, have high quality requirements for paint appearance. A common type of paint condition is orange peel which comprises roughly sinusoidal surface undulations or bumps greater than 0.5 mm diameter. To determine acceptable quality of a painted product it is necessary to measure the severity of orange peel and compare it to known standards. The common practice for measuring orange peel is to subjectively compare standard panels with the article being inspected. Such subjective inspection is marked by inconsistency due in part to different inspectors. There have been some instruments proposed to measure orange peel.
Another measure of paint surface quality is distinctness of image or DOI. This measures a composite of orange peel, texture and gloss. Texture and gloss refer to surface roughness smaller than orange peel and extending into the microscopic range. To determine DOI the clarity of reflection of a standard pattern is graded by subjectively observing the reflection and assigning a figure of merit to the surface in accordance with the observer's judgment of the distinctness of the image. An instrument for measuring the DOI of a surface is disclosed in the U.S. Pat. No. 4,527,898 to Stapleton. That patent discloses a meter in contact with the surface for projecting a light onto the surface and sensing the reflected beam by a detector, chopping the beam by a motor driven blade, and a circuit for determining the rate of change of the detector signal, the rate of change being a measure of DOI. A table is used to correlate the measured rate of change to the subjective DOI scale.
It is desirable to use instruments to obtain objective paint quality measurements. Such instruments have limited usefulness when they require contact with the surface. When applied to automotive vehicles it is advantageous to make the surface measurements from a remote location so that the instrument does not interfere with other activities and allows vehicle motion during the measurement. In addition it is preferred that continuous or repetitive measurements ensue as the vehicle passes through an inspection station.